The present invention relates to novolac resins, and more particularly to fast curing novolac resins and shell molding compositions used in the production of foundry shell cores and molds.
Foundries today use resin-coated refractory granules such as sand to form shell molds and cores used for casting metal and other materials. Typically, a minor proportion of resin and hardening agent are combined with a clean, dry sand and mulled until the resin is uniformly dispersed over each grain of sand. The resin-coated sand, or shell molding composition, is then stored for use in the production of shell molds and cores.
Novolac resins have found high acceptability in the production of shell molding compositions for making shell molds and cores. Conventionally, an acid-catalyzed phenol-formaldehyde novolac resin is employed to coat the sand. Prior to coating, an accelerator such as hexamethylenetetramine (hereinafter "hexa") is mixed with the novolac resin to function as a catalyst and source of additional formaldehyde to cross link the novolac resin. Such conventional novolac resins are methylene chained polyphenylols in which the methylene linkages are attached at the o-o', o-p' and p-p' positions to the adjacent phenylol groups. A typical distribution of the methylene linkage attachment locations to the adjacent phenylol groups are about 10% o-o', 45% p-p' and 45% o-p', based on the total number of methylene linkages present in the resin. The distribution of the attachment locations of the o-o' linkages may vary in the conventional acid-catalyzed novolac resins by no more than four percent from the foregoing percentages, while the p-p' and o-p' percentages may vary slightly more.
Conventional shell molding compositions produced from such conventional novolac resins function as an excellent binder for sand in the production of shell molds and cores. Conventional shell molding compositions, however, exhibit a relatively long cure time, i.e., slow cure speed, resulting in a relatively long residence time for the curing mold or core in the machines that produce the molds and cores. For some time, the industry that supplies resin for shell molding compositions has searched for a faster curing resin suitable for use in shell molding compositions. For example, accelerators for the reaction between hexa and the novolac resins have been investigated. The use of accelerators such as ammonium salts of carboxylic acids, resorcinol, orcinol, phloroglucinol and thiourea has resulted in a cure speed increase; however, the tensile strength and melt points of the resulting cured shell mold or core are reduced to unacceptable levels when such accelerators are present.
It is known that novolac resins containing a high proportion of o-o' methylene linkages are fast curing plastic molding compounds. See for example Bender et al., U.S. Pat. No. 2,475,587, and Frazier et al., J. Appl. Chem.7, December 1957, "Preparation of `High Ortho` Novolac Resins I, Metal Ion Catalysis and Orientation Effect," page 676, and "Preparation of High Ortho Novolac Resins II, The Course of the Reaction," id, page 689. The use of such resins containing a large proportion of o-o' methylene linkages (hereinafter "high ortho resins") has been suggested as a substitute for conventional novolac resins to increase the cure speed of shell molding compositions. Although the high ortho resins, when substituted for conventional resins, do increase the cure speed of shell molding compositions, a detrimental reduction in the tensile strength of the resulting mold or core is exhibited. Thus high ortho novolac resins have not gained wide acceptance in the shell molding industry as a substitute for the slower curing, conventional resins.
Accordingly, it is a broad object of the present invention to provide a novolac resin and a shell molding composition produced from the resin that has a cure speed greater than that of conventional resins and shell molding compositions. It is a further object of the present invention to provide such fast curing resins and shell molding compositions that do not produce molds and cores that exhibit a significant loss in tensile strength, but rather provide a mold or core that has a tensile strength about equal to or greater than that of the molds and cores formed from conventional shell molding compositions.